Many systems using video image signals or television/NTSC image signals (hereinafter collectively referred to as "video" signals) involve the conversion of analog signals to digital signals for recording, processing, transmitting and/or storing the images. Similar processes, also requiring analog to digital conversion, may also take place with respect to audio signals. Although digital processing can be used to remove or correct for certain noise levels once analog to digital conversion has taken place (regardless of whether video or audio signals are involved), additional circuitry and processing power is required which results in higher costs and increased complexity of circuit design and programming. The costs and complexity continue to rise as the signal-to-noise ratio of analog signals increase.
Improvements in image pick-up devices, such as charge coupled devices (CCDs), have made conversion of analog signals to digital signals more difficult and more expensive. One factor is that as the signal-to-noise ratio (SNR) approaches and exceeds seventy (70) db, ADCs with adequate quantization accuracy (particularly in the small signal, critical region) become more difficult to build.
The effect of noise on small amplitude analog signals is greater than the effect of noise on large amplitude analog signals. This is particularly evident as the amplitude of the small signal and the amplitude of the noise signal are closer in magnitude to each other. Compounding the noise problem for small signals, ADCs tend to add "channel noise", as do other digital processing components and systems. The addition of channel noise can be significant in reducing the small signal, signal-to-noise ratio. As the small signal, signal-to-noise ratio approaches 1:1, the small signal becomes effectively lost in the noise signal. Removing or reducing noise at that point is difficult. Even then, it is possible to lose part(s) of the small signal in trying to remove or reduce the noise component.
It is understood that noise is more visible and has a greater detrimental effect (in terms of perception) on small (low amplitude level) video or audio signals than on large (high amplitude level) signals. Thus, adding a noise signal with higher amplitude to a large signal may achieve the same level of perceived signal degradation by a viewer or listener as compared to adding a noise signal with a smaller amplitude to a small signal.
In order to reduce perceivable quantization noise, it is desirable for analog video signals to be encoded with more bits in the low level regions when quantizing an analog signal during A/D conversion. Small signals are viewed as the "critical region" since noise is more noticeable for the smaller signals than the larger signals. In video applications, small signals provide black or dark color while large signals provide white or saturated color.
Merely increasing the gain of analog input signals across the entire dynamic range of the input signal does not solve the problem of additional noise from ADCs. If the gain increase required to effectively minimize the noise added by the A/D conversion is large enough, the corresponding gain increase to the large signal may yield too great a large signal amplitude. At this point clipping and loss of signal information may occur.
When carrying out noise reduction on multi-component signals such as R,G,B video signals (a three component signal comprised of red, green and blue signals) or stereo audio signals (a two component signal comprised of right channel and left channel signals) it is important to maintain the relationship of each component of the multi-component signal to the other component(s) of the multi-component signal. If noise reduction (gain change) is not carried out in the same amount on each component of the multi-component signal, the final signal may be adversely affected from the perspective of the viewer/listener. For example, if the red, blue or green components of an R,G,B video signal are individually modified, the colorimetry or chromaticity may be adversely affected. In the case of a stereo audio signal, the signal balance between right and left channels may be adversely affected.